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Originally Posted by Augi
It doesn't suddenly stop, either. The mechanics of every *-ROM I have owned (to confess, only four) have gentily slowed down the disc. It wouldn't make sense if it did stop it suddenly, and it certainly doesn't make sense that the motor locks without power. There is a back-EMF in the motor if a closed circuit exists, counter-acting the rotation. In my honest opinion, it would not be strong enough to cause such a deceleration to make a disc warp thus contacting any stray parts in the ROM.
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The motor doesn't lock, however the dc electric motors typically used in this application apply resistance when not active; it is, as you noted, inherent in the design. I wouldn't really be concerned about the disc warping so much as any shuddering or vibration. I'm not sufficiently invested to actually crunch the numbers, but I doubt the amount of energy we're talking here is sufficient to cause any appreciable warp in the disc, even in the fastest cd-rom drives on the market (let alone your standard cd player, which rotates at a much lower speed). However, the disc does undergo a deceleration as the sudden cut in power to the motor results in an abrupt change from a constant rotational velocity to a relatively rapid move towards rest. I should think most of the kinetic energy of the rotational mass would be dissipated through heat in the motor, but it is possible that some of it could be redirected into a vertical acceleration of the disc (ie a vibration). Given that most players give the disc a fair amount of vertical play it is possible that a poorly designed player with an excessively high deck and/or any components extending toward the disc could result in the disc contacting said deck (and/or components) which, in turn, could result in a scratch. The probability of this, however, is quite low, as noted above.